Secondary batteries are rechargeable and can be compact in size and have high capacity. Recently, the demand for portable electronic devices, such as camcorders, portable computers, and mobile phones, has increased, leading to active research into and development of secondary batteries. Representative examples of widely-used secondary batteries include nickel metal hydride (Ni-MH) batteries, lithium (Li) polymer batteries, and lithium ion (Li-ion) batteries.
Since lithium has a small atomic weight, secondary batteries using lithium can have high capacitance per unit weight. However, since lithium readily reacts with water, lithium-based secondary batteries generally use non-aqueous electrolytes. In such lithium-based secondary batteries, there is no water restricting the charging and discharging voltage, enabling electromotive forces of about 3 to 4 V to be obtained. Lithium ion secondary batteries are an example of such lithium-based secondary batteries.
The non-aqueous electrolytes used in lithium ion secondary batteries are generally classified into liquid electrolytes and solid electrolytes. Liquid electrolytes are manufactured by dissolving a lithium salt in an organic solvent. The organic solvent, for example, may be an alkyl carbonate such as ethylene carbonate or propylene carbonate, or may be another organic compound.
One problem with these electrolytes, however, is that they typically have low ion conductivity. To compensate for the low ion conductivity of these electrolytes, the areas of the electrode activation materials have been increased, creating an overlap between the electrodes.
However, increasing the overlap between the electrodes limits battery performance in several ways. For example, the low ion conductivity of the electrolyte causes high internal impedance of the battery, resulting in an increased voltage drop in the battery. In particular, the low ion conductivity of the electrolyte causes a reduction in battery current, thereby also reducing battery power.
In addition, movement of lithium ions is limited by a separator which is positioned between the two electrodes. If the separator is not sufficiently permeable and wettable to the electrolyte, the movement of lithium ions is substantially inhibited. As a result, the electrical properties and performance of the battery may deteriorate.
Therefore, in addition to heat resistance, heat deflection resistance, chemical resistance, and mechanical strength, the cross-sectional pore ratio of the separator and the wettability of the separator to the electrolyte are important characteristics of the separator for determining battery performance. The cross-sectional pore ratio of a porous object (e.g., the separator) is the ratio of pore areas to the area of the cross section.
The separator serves as a safety device for preventing the battery from overheating. If the temperature of the separator increases beyond a predetermined level due to some abnormalities in the battery, the polyolefin-based porous membrane of the separator becomes soft and partially melts. This causes the pores of the porous membrane to close. The pores function as passages for the electrolyte solution, and more specifically, for lithium ions. When the flow of lithium ions stops, the current flow between internal and external portions of the battery becomes blocked, thereby slowing or stopping the temperature increase in the battery.
However, the temperature in the battery can suddenly and continuously increase for any reason, for example external thermal transfer. When the temperature increases for more than a predetermined time interval, the separator may melt and destruct irrespective of the shutdown of the pores. This creates partially melted portions in the separator, and these partially melted portions may cause the two electrodes of the battery to be in contact with each other, thereby forming a short circuit. Alternatively, the separator may contract, creating contracted portions in the separator. These contracted portions may also cause the two electrodes of the battery to be in contact with each other, forming a short circuit. Short circuits cause serious harm to the battery.
In a high capacitance secondary battery, a large amount of current can flow over a short period of time. When excessive current flows in such a battery, the temperature in the battery cannot be decreased by shutting down the pores of the separator and blocking current flow. Furthermore, the heat generated by such excessive current flow may cause the separator to continue to melt and destruct. As a result, a short circuit due to the destruction of the separator becomes increasingly possible.
Although blocking the current flow by shutting down the pores of the separator is important, a countermeasure against melting and contraction of the separator is also important to prevent the battery from overheating. Therefore, a need exists for a separator which prevents short circuits between the electrodes at high temperatures, for example, at 200° C. or higher.